246 DESCRIPTIONS OF PREPARATIONS. 



tube are composed of a delicate supporting lamina separating a layer of 

 ectoderm and endoderm cells, the latter being ciliated. Consequently the 

 products of digestion, a process which goes on only in the hydranths, are 

 conveyed along the coenosarcal tube through branches, stems, and roots, 

 in brief, through the whole hydrophyton. The ectoderm of the coenosarc 

 is separated from the perisarc by a space, but the ectoderm of each hydranth 

 remains in contact with its chitinous theca at the base of the cup. Wherever 

 growth is taking place, i.e. at the tip of a root, or in a developing hydranth, 

 the perisarc is in close contact with the ectoderm by which it is formed. 



There may be noted in this specimen, towards the tip of the branches 

 and upon the side turned uppermost, certain pear-shaped bodies, ihegeno- 

 thecae, gonangia, or capsules. Each gonangium contains a modified hy- 

 dranth, the blastostyle, which bears the generative zooids. These zooids 

 are never set free in the family Sertularidae, and have, as in the family Plu- 

 maridae., with few exceptions, the structure of a medusoid gonophore. The 

 fertilised ovum in Sertularia abietina and some of its allies^ passes into a 

 small cyst, the acrocyst, formed at the apex of the gonangium. Here it 

 undergoes development and is set free as a ciliated Planula which, after 

 leading a wandering life, settles down, and by growth and budding es- 

 tablishes a new colony. 



The hydranth resembles Hydra in all essentials (see Plate xiv. Fig. 6, 

 and description). Like that organism it consists of a hydrocephalis ( = oral 

 and stomachal regions) and a peduncle or hydrocope which is very short. 

 The hypostome or oral cone is conical, the tentacles filiform and arranged 

 in a single circle. They are solid and their axis is composed of a single row 

 of vacuolated endoderm cells. The blastostyle is borne on a short peduncle 

 or gonocope. 



Representatives of two other Phyla may be seen to have affixed 

 themselves to the main stems of this specimen. One of them is a 

 Spirorbis, a small Tubicolar Annelid with a discoidal shell somewhat like 

 that of the freshwater mollusc Planorbis : the other is a Cyclostomatous 

 Polyzoan, Diastopora Patina, which with its aggregated calcareous cells 

 presents an appearance not unlike that of a small tubiflorous flower belong- 

 ing to a plant of the order Compositae. Coelenterata with an external 

 perisarc and a single circlet of tentacles bear a superficial resemblance to 

 many Polyzoa, a group formerly classed with them. A Hydroid may how- 

 ever be readily distinguished at once from a plant-like Polyzoan by the 

 presence of thread-cells on the tentacles as well as by other anatomical 

 features. Compare Fig. 10, p. 235, ante, and description. 



The order Hydroidea is divisible into the sub-groups (i) Hydrocorallina, 

 colonial organisms with an external calcareous skeleton; (2) Tubulariae or Gymno- 



